Lewy body diseases (LBDs) are characterized by degeneration of the dopaminergic system, motor alterations, cognitive impairment, and formation of Lewy bodies (LBs). (McKeith et al., Clinical and pathological diagnosis of dementia with Lewy bodies (DLB): Report of the CDLB International Workshop, Neurology (1996) 47:1113-24). LBDs include Parkinson's disease, Diffuse Lewy body disease (DLBD), Lewy body variant of Alzheimer's disease (LBV), and combined PD and Alzheimer's disease (AD) and the syndromes identified as multiple system atrophy (MSA). Dementia with Lewy bodies (DLB) is a term coined to reconcile differences in the terminology of LBDs. Disorders with LBs continue to be a common cause for movement disorders and cognitive deterioration in the aging population (Galasko et al., Clinical-neuropathological correlations in Alzheimer's disease and related dementias. Arch. Neurol. (1994) 51:888-95). Although their incidence continues to increase creating a serious public health problem, to date these disorders lack approved treatments (Tanner et al., Epidemiology of Parkinson's disease and akinetic syndromes, Curr. Opin. Neurol. (2000) 13:427-30). The cause for LBD's is controversial and multiple factors have been proposed to play a role, including various neurotoxins and genetic susceptibility factors.
AD, PD, and DLBD are the most commonly found neurodegenerative disorders in the elderly. Recent epidemiological studies have demonstrated a close clinical relationship between AD and PD, as about 30% of Alzheimer's patients also have PD. Compared to the rest of the aging population, patients with AD are thus more likely to develop concomitant PD. Furthermore, PD patients that become demented usually have developed classical AD. Although each neurodegenerative disease appears to have a predilection for specific brain regions and cell populations, resulting in distinct pathological features, PD, AD, and DLBD also share common pathological hallmarks. Patients with familial AD, Down syndrome, or sporadic AD develop LBs on the amygdala, which are the classical neuropathological hallmarks of PD. Additionally, each disease is associated with the degeneration of neurons, interneuronal synaptic connections and eventually cell death, the depletion of neurotransmitters, and abnormal accumulation of misfolded proteins, the precursors of which participate in normal central nervous system function. Biochemical studies have confirmed a link between AD, PD and DLB.
In recent years, new hope for understanding the pathogenesis of LBD has emerged. Specifically, several studies have shown that the synaptic protein alpha-synuclein plays a central role in PD pathogenesis since: (1) this protein accumulates in LBs (Spillantini et al., Nature (1997) 388:839-40; Takeda et al., J. Pathol. (1998) 152:367-72; Wakabayashi et al., Neurosci. Lett. (1997) 239:45-8), (2) mutations in the alpha-synuclein gene co-segregate with rare familial forms of parkinsonism (Kruger et al., Nature Gen. (1998) 18:106-8; Polymeropoulos, et al., Science (1997) 276:2045-7) and, (3) its overexpression in transgenic mice (Masliah et al., Science (2000) 287:1265-9) and Drosophila (Feany et al., Nature (2000) 404:394-8) mimics several pathological aspects of PD. Thus, the fact that accumulation of alpha-synuclein in the brain is associated with similar morphological and neurological alterations in species as diverse as humans, mice, and flies suggests that this molecule contributes to the development of PD.
The neuritic plaques that are the classic pathological hallmark of AD consist essentially of amyloid beta (Aβ) peptide, an amino acid proteolytic product of the amyloid precursor protein (APP), and NAC, a 35 amino acid proteolytic fragment of alpha-synuclein. Both Aβ and NAC were first identified in amyloid plaques as proteolytic fragments of their respective full-length proteins, for which the full-length cDNAs were identified and cloned. (Iwai A., Biochim. Biophys. Acta (2000) 1502:95-109); Masliah et al., AM. J. Pathol (1996) 148:201-10; Ueda et al., Proc. Natl. Acad. Sci. USA (1993) 90:11282-6).
Alpha-synuclein is part of a large family of proteins including beta- and gamma-synuclein and synoretin. Alpha-synuclein is expressed in the normal state associated with synapses and is believed to play a role in neural plasticity, learning and memory. Mutations in human (h) alpha-synuclein that enhance the aggregation of alpha-synuclein have been identified (Ala30Pro and Ala53Thr) and are associated with rare forms of autosomal dominant forms of PD. The mechanism by which these mutations increase the propensity of alpha-synuclein to aggregate are unknown.